DC Flex installation 1000Amp shunt required [RE-wrenches]

Tue Feb 5 10:45:17 PST 2008

Tump - 

Topica seems to be down again - so I am reposting this - pardon me if it
shows up twice... 

There are several ways this can be made to work - 

1) Use a single 500 amp shunt for the four inverters.  The actual 500
amp rating of a shunt is its calibration level - not a true current
limit as with a cable.  The amount of current a shunt can carry depends
on the ambient temperature and the amount of heat sinking added to the
shunt by the inverter and battery cables and shunt busses connected to
it.  

I have operated 500 amp shunts at levels as high as 650 amps with four
4/0 cables and two of the OutBack brass shunt busses for several hours
without issue.  Even if the shunt is overloaded it really only affects
its calibration - I have never seen a 500 amp shunt fail (yet).  

With battery systems these extremely high current levels usually can not
be maintained for very long - the battery discharges, often in an hour
or so.  So the thermal mass of the cabling helps to limit the shunt
temp.  If you have massive charging sources over around 500 amps then
this could be an issue - but that is not likely.  

You mentioned that this is a 24 vdc system with four VFX3524 inverters
and each has a 250 amp breaker - if you went to 48 vdc then you could
use  the 125 amp breakers ~ 4 x 125 amps = 500 amps.  

2) Use two DC current shunts for the inverters/loads.  This can be done
with the FNDC < BUT > you have to have each of the shunts connected to a
separate channel of the FNDC - resulting in only one channel remaining
for the DC sources.  The shunts can be wired either in true parallel
(bussed together on both sides) or they can be connected just on the
battery side and run each set of inverters (L1 and L2) separately.  

The hydro and PV charging sources can be ran together on the same shunt
- and if you are using MX60 or FM80 controllers the data logging can be
used to separate out what the PV did versus the hydro.     

3) You can use a 1000 amp 100 millivolt shunt - which are available from
the shunt manufacturers as a special order item.  Be sure it's a 100 MV
and not a 50 MV however - it needs to have the correct ration of 1
millivolt per 10 amps.  The problem with these shunts is that they are
huge - about three times the size of a 500 amp shunt - and they have
single 1/2 inch bolt connections on each side compared to the 3/8 inch
bolt connections we usually use.  Some only offer these larger shunts as
500 amp 50 mv versions -  it would actually be acceptable - its really a
matter of increasing the surface area of the shunt to dissipate the
higher heat produced is all.  

We have looked at having a custom built 1000 amp shunt made with dual
3/8 inch bolts on each side and the same mounting hole locations as the
500 amp shunt - but the other two options seemed to work out well so far
for most customers. If there is enough interest then we will make it
happen.  I'll check to see what available "off the shelf" as well.   

Let me know what you think...

Christopher

Christopher Freitas
Director of Research of Development 
OutBack Power Systems, Inc.
cfreitas at outbackpower.com
Tel 360 435 6030
cell 360 202 4239
19009 62nd Ave NE 
Arlington WA 98223 USA
www.outbackpower.com

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